[18F]Tosyl fluoride as a versatile [18F]fluoride source for the preparation of 18F-labeled radiopharmaceuticals

Positron emission tomography (PET) is an in vivo imaging technology that utilizes positron-emitting radioisotope-labeled compounds as PET radiotracers that are commonly used in clinic and in various research areas, including oncology, cardiology, and neurology. Fluorine-18 is the most widely used PET-radionuclide and commonly produced by proton bombardment of 18O-enriched water in a cyclotron. The [18F]fluoride thus obtained generally requires processing by azeotropic drying in order to completely remove H2O before it can be used for nucleophilic radiofluorination. In general, the drying step is important in facilitating the radiofluorination reactions and the preparation of 18F-labeled PET radiotracers. In this communication, we have demonstrated the feasibility of using [18F]tosyl fluoride ([18F]TsF) as a versatile [18F]fluoride source for radiofluorination to bypass the azeotropic drying step, and we have developed a continuous flow solid-phase radiosynthesis strategy to generate [18F]TsF in a form that is excellent for radiofluorination. [18F]TsF shows high reactivity in radiofluorination and provides the features suitable for preparing PET radiotracers on a small scale and exploring novel radiolabeling technologies. Thus, using [18F]TsF as a [18F]fluoride source is a promising strategy that facilitates radiofluorination and provides a convenient and efficient solution for the preparation of 18F-labeled radiopharmaceuticals that is well matched to the emerging trends in PET imaging technologies.

. The manual elution setup: A 12 mL syringe (left) was used to provide the reduced pressure for loading. Different tubes (middle) were used to collect water/acetonitrile and the final radioactivity via the cartridge from corresponding tubes (right), containing [ 18 F] (1 mg, 5.25 µmol) in the chosen solvent (0.5 mL) was used to elute the radioactivity, followed by rinsing with the chosen solvent (0.5 mL); b. KF (0.29 mg,4.99 µmol) was added to [ 18 F]fluoride; c. Radiochemical conversion (RCC) was determined by the eluted radioactivity over total radioactivity.  (1 mg, 5.25 µmol) in acetonitrile (0.5 mL) was used to elute the radioactivity, followed by rinsing with acetonitrile (0.5 mL); b. Resin (~50 mg) was treated by rinsing sequentially with acetonitrile (5 mL), water (5 mL), 1 M KHCO3 (10 mL) and water (5 mL); c. Radiochemical conversion (RCC) was determined by the eluted radioactivity over total radioactivity.  cartridge. The cartridge was rinsed with water (1 mL) and acetonitrile (5 mL), and then a solution of TsCl (1 mg) and TsOH·H2O (0.25 mg) in acetonitrile (0.5 mL) was circulated through the cartridge for the specified time at the specified flow rate. Finally, the cartridge was rinsed with acetonitrile (2 mL) and all the eluted solution was combined for HPLC analysis (n=2); b. The number is the peak integration/1000.   °C, 8 min. Cartridge was eluted with K2CO3/K222 (1:2 5 mg) in acetonitrile (300 µL) and water (200 µL), and the eluted solution was dried under a standard drying protocol (105 °C, N2, 3 × 1 mL acetonitrile). The reaction mixture (10 µL) was analyzed by analytical HPLC (Altima C18 250×4.6 mm, 60% acetonitrile/40% water/0.1% TFA, 2 mL/min, 240 nm, retention time = 4.05 min). The precursor contains the chlorine-derivative. The number is the integration/1000; b. The cartridge was pre-treated with 0.5M K2CO3 (10 mL) and water (15 mL). Radiochemical conversion (RCC) was determined by radio-TLC; b. UV and radioactivity profiles of reaction mixture.  Table S14. One-pot two-step model labeling reaction  From processing of sulfonyl fluoride Note: a. Radiochemical conversion (RCC) was determined by radio-TLC or radio-HPLC of the reaction solution. The percentage of radioactivity left in the reaction vessel over total radioactivity is noted in the parentheses.

General information for test radiolabeling
Method A: A fraction of [ 18 F]TsF elution (50-500 µL) was heated with K2CO3/K222 at 100-105 °C for 2-5 min, and then a solution of the radiolabeling precursor in acetonitrile was added. The test radiolabeling was carried out under the condition indicated in Table S16. Upon completion of the reaction, the mixture was analyzed by radio-TLC for RCC and by radio-HPLC to identify the major radioactive product. The radioactivity left in the reaction vessel was measured to determine the loss of radioactivity to the container.
Method B: The same as described above except that the test radiolabeling was carried out in one pot without the pretreatment step.

Radiosynthesis of [ 18 F]FDG-intermediate
The solution of [ 18 F]TsF was added to K2CO3/K222 (27.8 mg, 31.2 µmol), and then heated at 100 °C for 3 min. A solution of FDG precursor (25 mg, 52 µmol) in acetonitrile (0.3 mL) was added, and the mixture was heated at 84 °C for 7 min. RCC of the reaction is 93% according to radio-TLC analysis of the reaction mixture (Silica gel/ethyl acetate) and 95% according to radio-HPLC.
1.1% of total radioactivity was lost to the reaction tube.
The tube was shaken briefly, and the reaction was allowed at room temperature for 6 min before   The first injection is the animal dose and the second injection is the co-injection of authentic FFNP (HPLC condition: Altima C18 250 × 4.6 mm, 70% MeCN/30% water, 250 nm, 2 mL/min).  The first injection is the animal dose and the second injection is the co-injection of FTT. (HPLC condition: Altima C18 250 × 4.6 mm, 30% MeCN/70% water, 250 nm, 2 mL/min)

One-pot radiosynthesis of [ 18 F]FDHT (13)
Into a 9 mL Pyrex tube containing K2CO3/K222 (1.8 mg, 2.0 µmol) was added a solution of [ 18 F]TsF(1.6 GBq) in acetonitrile (0.5 mL). After the tube was capped and heated at 110 °C for 3 min, the tube was removed from the oil bath and allowed to cool down in 2 min. Into the tube was then added the FDHT precursor (0.9 mg) in acetonitrile (100 µL), and the reaction was allowed at room temperature for 7 min. After a solution of NaBH4 (5 mg) in ethanol (500 µL) was added, the reaction mixture was heated at 80 °C for 3 min, followed by the addition of 1N HCl (500 µL) and heated at 80 °C for 5 min. The reaction mixture was diluted with water (2 mL) and 1N NaOH ( was collected at 21-22 min, and further processed using the typical solid-phase extraction method to produce the final dose for animal study in 10% ethanol/saline.

Radiosynthesis of [ 18 F]FTHA (16)
Into a 9 mL Pyrex tube containing K2CO3/K222 (2.5 mg, 2.8 µmol) was added [ 18 F]TsF (1.41 GBq) in acetonitrile (1 mL). The tube was capped, heated at 105 °C for 2.5 min and then acetonitrile was removed under an argon flow at 105 °C, followed by the addition of the precursor (2 mg) in acetonitrile (0.5 mL). After the reaction was heated at 105 °C for 8 min, the solution passed through a silica gel plug (10 × 5 mm) to remove unreacted [ 18 F]fluoride, followed by rinsing the tube and the plug with acetonitrile (2 × 0.5 mL). For deprotection, 10% Pd/C (6 mg) and Et3SiH (10 µL) in methanol (0.5 mL) were added to the combined solution. After 7 min at room temperature, the reaction mixture passed through a 3 mL cartridge containing celite (10 mm) to remove Pd/C, followed by rinsing with methanol (2 × 0.5 mL).   The first injection is the animal dose and the second injection is the co-injection of FTHA (HPLC condition: Altima C18 250 × 4.6 mm, 90% methanol/10% water with 0.4% acetic acid, 230 nm, 2 mL/min).

Radiosynthesis of [ 18 F]FGLN (17)
The synthesis followed the reported procedure with some modification in the deprotection step. under pressure and the cartridge was rinsed with water (10 mL) to remove residual solvent. The radioactivity was eluted with ethanol (0.5 ml) and ethanol was then removed under an argon flow at 105 °C before the addition of 4 N HCl (200 µL) for de-protection. The mixture was heated at 105 °C for 7 min and then diluted with water (1 mL) and neutralized with 1 N NaOH to pH = 5-7.
The final dose was prepared by diluting with water to a total volume of 5.2 mL to achieve the concentration of sodium chloride (0.9%) for injection.   .

A typical procedure for SPE extraction
The HPLC fraction of a radioactive peak was collected and diluted with water (40 mL) in a 50 mL glass tube. The diluted solution was then passed through an SPE cartridge (Waters C18 light or HLB light) under pressure, applied with a 10 mL syringe, and the cartridge was further rinsed with water (10 mL) and dried with air (10 mL). The radioactivity was then eluted with ethanol in portion (0.1 or 0.2 mL), and the most concentrated portion was diluted with saline to prepare a final dose in 10% ethanol in saline. water/0.1% TFA, 2 mL/min, 228 nm).